The present invention relates to a self-calibrating oscillator for calibrating the frequency of an output signal.
RC oscillators are widely used in semiconductor products. This is because an RC oscillator is an inexpensive clock source and allows for the generation of various frequencies by changing resistance and the capacitance. However, the circuits of the oscillator produce a delay that affects the oscillation frequency. The time of such delay is determined in accordance with the power supply voltage, the environmental temperature, the manufacturing process, and other factors. Thus, for practical use, the RC oscillator is limited to low frequency and low accuracy applications.
Accordingly, an RC oscillator that oscillates with high accuracy has been developed (for example, refer to U.S. Pat. No. 5,594,388, page 1). U.S. Pat. No. 5,594,388 describes an oscillator including a resistor and capacitor and has a time constant determined by the resistance and capacitance. Prior oscillators used RC circuits for frequency generation. However, the oscillator of U.S. Pat. No. 5,594,388 does not use an RC circuit. Rather, this oscillator compares the RC time constant with a period obtained from a frequency generated by another oscillation circuit. Then, the oscillator self-calibrates the frequency of the entire RC oscillation circuit in accordance with the comparison result.
The oscillator described in U.S. Pat. No. 5,594,388, however, needs a charge pump to generate a voltage signal that calibrates the oscillation frequency. In addition, the oscillator is continuously self-calibrating the frequency, which consumes a lot of power.